Embodiments of the invention generally relate to devices, systems, and methods for recovering metallic substances such as solder from secondary sources such as dross. More particularly, the invention relates to systems, devices, and methods for recovering solder from dross during wave soldering processes.
When many molten metals are in contact with the air, compounds of those metals, primarily oxides, but also nitrides and other non-metallic impurities, can be formed. When the molten metal is moving, the metal and the metal oxide combine to form a material known as dross. Dross consists of droplets of the molten metal that are encrusted with the oxide to form a sponge-like network. The dross of lead-tin solder, for example, can have a lumpy, granular appearance. The dross floats, because of surface tension and/or buoyancy, on the molten metal or sticks to the container holding the metal. Various methods have been used to attempt to prevent formation of dross, such as by preventing air from contacting the solder by pouring a layer of oil on the solder. The use of oil can be messy, however, and may degrade the solder.
Many methods of soldering, such as in wave soldering, can cause dross (also referred to as a solder-dross mixture) to be formed. For example, in a wave soldering machine, solder in a bath is pumped to create a static wave in the surface of the solder, and printed circuit boards are fed across the crest of the wave so that the solder xe2x80x9ckissesxe2x80x9d the leads of the components and the tracks of the circuit board. If dross is allowed to build up, it can become entrained in the wave and adversely affect the quality of the soldering, causing low product yields. In addition, the dross can self skim from the wave to collect on the surface of the solder pot in the wave-soldering machine. When dross recirculates through the solder pump, it can appear as undesirable matter in the wave. In addition, leaving dross in the solder pot can create problems for soldering quality and solder pot life.
Frequent removal of dross can be necessary for proper use of the solder, especially in wave soldering machines. One removal method used in wave-soldering machines involves skimming the dross or in some other way removing the dross from the surface to avoid recirculation of dross. One method of removing dross from the solder is by ladling off the dross (which floats). A large percentage (e.g., 30-90%) of the solder, however, can be lost with the dross. Further, disposing of the raw dross and replacing it with xe2x80x9ccleanxe2x80x9d metal can be expensive.
It is estimated that dross can consist of 30-90% good solder capable of being reclaimed. One method of reclaiming solder from dross is heating the dross that has been skimmed/ladled off. However, this method can be inefficient and uneconomical. Because dross consists of pockets of good solder that are surrounded by a crust-like envelope of tin and lead oxides, some solder reclamation methods attempt to compress the dross, sometimes while heating the dross, until the oxide bursts and the liquid solder can escape.
One dross compression technique involves using a pair of rollers disposed in parallel, and rotated in opposing directions to form a xe2x80x9cbitexe2x80x9d on dross directed therebetween. The dross is fed between the rollers, and liquid solder and sheets of dross are produced at the other side of the rollers. This dross compression technique does not always efficiently separate liquid solder from the dross sheets, and the resultant liquid solder and/or dross sheets can be impure. In addition, the sheets of dross can be awkward to handle and can require frequent maintenance to remove. Further, the sheets of dross may still contain usable solder. To help divide the sheets into a more manageable size and guide the liquid solder away from the dross, the rollers may be ribbed, grooved, or perforated.
The inventor of the present invention has found that flowing a solder-dross mixture along rollers having channels, indents, holes, and other non-smooth surfaces can create additional dross in the solder-dross mixture at the location of the hole, indent, etc. The inventor of the present invention has further found that methods of reclaiming solder that use devices (e.g., knife-like devices, augers, and the like) to cut through dross floating on the surface of the solder (such as solder in solder pots) also can create even more dross in the solder-dross mixture. The inventor of the present invention has further determined that providing at least one roll with a smooth surface can decrease the dross created in the solder-dross mixture passed between a pair of rolls, which may help to increase the solder reclaimed from the solder-dross mixture.
In addition, the inventor of the present invention has found that, when compressing dross through a pair of rolls, the rolls can be spaced so that the compressions produces dross powder and liquid metal, instead of a sheet of dross. The inventor of the present invention has further discovered that heating at least one of the pair of rolls can help the solder-dross mixture separate into liquid solder and dross powder, with dross powder tending to cling to the heated roll because of the heat of the roll and/or the inherent surface tension of the dross powder. This dross powder drawn to the heated roll can then be removed from the roll, such as by scraping or by a vacuum, enabling the remaining liquid solder to be re-used.
In one embodiment, the invention includes an apparatus that separates solder from solder dross, comprising first and second rolls and a fixture. The first roll has a substantially cylindrical surface and is rotatable about a first longitudinal axis. The second roll has a substantially cylindrical surface and is rotatable about a second longitudinal axis parallel to the first longitudinal axis. The fixture is coupled to the first and second rolls and spaces the second roll apart from the first roll by a first distance, such as 0.001 to 0.015 inches, the first distance sufficient to separate a solder-dross mixture directed between the first roll and the second roll into a liquid solder and a dross powder when the first and second rolls are rotated in opposite directions.
The first roll can be constructed and arranged to operate at a temperature sufficient to cause the dross powder to be drawn toward the surface of the first roll, and the second roll can be constructed and arranged to operate at a second temperature that is lower than the first temperature. The surface of the first and second rolls can be substantially smooth (e.g., no rougher than 128 xcexcin. RMS). The fixture can be constructed and arranged to hold the first and second rolls in an alignment such that the first and second longitudinal axes are parallel and lie in a common plane that is positioned no more than forty-five (45) degrees from horizontal.
In one embodiment, the apparatus further comprises a dross remover, such as a scraper, constructed and arranged to remove dross powder from the first roll. In one embodiment, the apparatus further comprises a container constructed and arranged to receive the liquid metal and dross powder. The container can maintain the liquid metal received therein in a liquid form and can be constructed and arranged to permit the liquid metal received therein to be directed into a first receptacle and the dross powder received therein to be directed into a second receptacle. The container can, for example, be the solder pot of a wave-soldering machine.
In one embodiment, the invention provides a system that separates solder from a solder-dross mixture contained in a first container, such as the solder pot of a wave soldering machine. The system comprises a dross mover and a solder separation apparatus. The dross mover, such as a conveyor, transports the solder-dross mixture from the wave-soldering machine to a solder separation apparatus, which comprises first and second rolls and a fixture. The first roll is constructed and arranged to be controlled a first temperature and has a substantially cylindrical surface and is about a first longitudinal axis. The second roll has a substantially cylindrical surface and is rotatable in a second direction about a second longitudinal axis parallel to the first longitudinal axis. The fixture is coupled to the first and second rolls and spaces the second roll apart from the first roll by a first distance, the first distance sufficient to separate a solder-dross mixture directed between the first roll and the second roll, when the first and second rolls are rotated in opposite directions, into a liquid solder and a dross powder when the first and second rolls are rotated in opposite directions.
In another embodiment, the invention provides, for a fixture having first and second parallel cylindrical rolls spaced apart by a fixed distance, a method for separating metal from a solder-dross mixture. A flow of a metal-dross mixture is delivered between the first and second parallel cylindrical rolls. The first and second rolls are rotated in opposite directions to draw the metal-dross mixture between them. The rotation of the first and second rolls applies a pressure to the solder-dross mixture that is sufficient to separate the solder-dross mixture into a liquid solder and a dross powder. The first roll can be heated to a temperature that causes the dross powder to be drawn to the surface of the first roll. At least a portion of the dross powder on the surface of the first roll can be removed.
In another embodiment, the invention provides an apparatus for separating metal from metal dross, comprising an input for receiving metal dross in a non-solid form and means coupled to the input for separating the metal dross into liquid metal and dross powder. This embodiment can further comprise means for removing the dross powder, and means for directing the liquid metal to a location where the liquid metal can be collected independent of the dross powder
Details relating to this and other embodiments of the invention are described more fully herein.